This invention relates to a rotation control valve of a power steering system comprising a spool section on an input shaft, and a valve body loosely fitted over the spool section and secured to an output shaft connected to the input shaft through a torsion bar to be operative to control supply and discharge of pressure fluid to and from a hydraulic cylinder acting on the output shaft in accordance with a relative angular displacement between the spool section and the valve body commensurate with a load applied to the output shaft.
In this type of rotation control valve of the prior art, it has hitherto been usual practice to mount the valve body in a valve housing and to fit a resilient O-ring formed as of rubber in an annular outer groove formed on an outer side surface of the valve body, to ensure that a hermetic seal is provided between supply and discharge ports for the pressure fluid formed in the valve casing and between the pressure fluid supply and discharge ports and the valve body and valve housing. In the rotation control valve of this construction, the resilient O-ring rotates with the valve body because the latter rotates in conjunction with the output shaft, so that the resistance offered to the rotation of the valve body increases. Also, an increase in the pressure of the pressure fluid increases the resistance offered by the O-ring itself. Thus difficulties have been experienced in letting the relative positions of the valve body and the spool section correspond to the load applied to the output shaft. This defect is important because it spoils the feel of a driver who operates a steering wheel by utilizing the power steering system.